Process and apparatus fob digesting



O :t. 6, 1936. T. L. DUNBAR PROCESS AND APBARATUS FOB- DIGESTING FIBROUS MATERIALS Original Filed April 21, 1930 UNH Awww

gaa/vento@ v I: L. unar wenn? Reissued Oct. 6, 1936 UNITED STATES PATENT OFFICE PROCESS AND APPARATUS FOB DIGESTING FIBBOUS MATERIALS Thoma: L. Dunbar, Watertown. N. Y., assigner to Chemipulp Process Ine., Watertown N. Y., a corporation nl New York Claiml. (CL 92-11) 'I'he present invention pertains to the digestion of iibrous material such as wood chips for example, to be used in the making oi paper or the like, and the main object of the invention is to provide an improved system for recovering acid gases and the heat units thereof, and to thereby reduce the consumption oi steam and to shorten thev time required for the digestion of such ilbrous materials.

During the greater part of the lite of the industry, after chips or the like have been deposited in the digester, cold acid liquor was introduced into the digester to saturate the ilbrous material. Afterwards, steam was turned into the bottom of the digester, and as the contents were heated up. gases and vapors were evolved, and these were led o to some suitable point of disposal. Such a system required the use of steam from the beginning to substantially the end of the cook, and permitted the waste of the gasl and the heat units impounded therein. Later, proposals were made to recover the gases and to employ the same to strengthen and heat raw acid liquor in an accumulator, and from the latter, the preheated and preconditioned liquor was fed into a digester containing raw brous material. Even in these later proposals, the digester was only partially iilled with liquor and chips, and a gas space remained above the liquor level.

In a still further proposal, after the chips have been placed in the digester, and the top of the latter has been sealed by means including a pressure operated valve. the preheated and preconditioned liquor. is forced into the digester until the pressure becomes suillcient to open the pressure valve and relieve the digester of the pressure. However, in that arrangement, there is no provision for venting o air into the atmosphere during the time that the digester is being illled with liquor, and as air has a deleterious effect on the acid, such as producing insoluble suliatesl this `character prevents the digester from being' entirely or completely filled with liquor, due to the fact that air and gases are trapped at the top of the digester.

Another object of my invention is to provide a system whereby a digester may be charged with chips to a level in close proximity to the top oil the digester, and then preconditioned liquor heated to a temperature of 40 C., or more, may be pumped into the digester. As the liquor rises in the digester, air is relieved from the top of the latter, and as soon as the SO: appears, (which can be detected by the odor oi the same) the air vent is closed and the `gas is diverted into the acid liquor storage tank to be absorbed by the acid liquor. Any gas remaining unabsorbed is returned from the storage tank to the suction side vof the liquor pumps in the acid producing system. As soon as the digester has been relieved oi' the gas during the illling operation, and liquor begins to ow from the top of the digester, the top of the digester is sealed, and then the pump used for lling the digester with liquor is continued in operation until a pressure of up to 300 pounds per square inch is pumped on the contentsoi' the digester. Obviously, this causes the liquor to penetrate the chips and the hot acid puts into partial solution the non-cellulose components of the raw material, and this allows a more rapid and thorough penetration oi the raw material.

Another object is to save the SO2 gas Just as soon as it appears at the top of the digester. As I completely ll the digester with acid at a temperature oi' fromv 40 C., to 135 C., the S02 gas and/or liquor appears at the top of the digester considerably-sooner than when nlling -with cold acid.

Another object of the invention is to furnish a system in which the digesters are provided with 'a liquorrelief tank which may receive gas `and/or liquor relieved from one or more of the digesters, this strong hot liquor being used inta subsequent cook to pretreat the fibrous material as it is intail, illustrated in the accompanying drawing, and

more particularly pointed out in the appended claims.

The accompanying drawing shows in side elevationthe novel apparatus forming part of my system.

In this drawing. I designates a digester, 2 the conventional low pressure relief header, and 3 the high pressure relief header. The headers, as is customary, extend along the upper portions of a series of digesters, but as the present invention may be explained in connection with a single digester, only one digester has been illustrated, it being understood that the invention may be K used with one or more such digesters.

In accordance'with this invention, a valved low pressure branch pipe I connects the top portion of the digester to the header 2, but for the purpose of the invention, this branch is furnished with an air outlet vent pipe 5, having a control valve 8. y

The top of the digester is also connected by valved branch pipes 50 and 1, to the high pressure header 3, and I again depart from the conven-l tional by connecting the valved side relief branch pipe 8 to the pipe 1 by means of a by-pass 3.

In order that relief fluids from the header 3 may be utilized to preheat and strengthen the acid liquor, the header 3 is provided with a conduit iB whlch communicates with the down pipe II that terminates in a distributor header Ila; valved branches ilb being led from this distrib- Autor header into the accumulator at predetermined points, these valved branches being located in elevation near the bottom of the accumulator I2. The, number of valved branches and their exact 'location is determined by the size of the accumulator, and for small accumulators, I have 'found that it is only necessary to use one nozzle entrance, but for larger sizes, it is sometimes vnecessary to use as many as flve.

Acid liquor is fed to the accumulator from a storage tank Il that is provided with an outlet pipe I5 leading to the suction side of a pump I8, and having a control valve I1; The pump forces the liquor through a riser I3 which empties into the upper portion of the accumulator and is provided at its upper portion with a controlA valve I3. In this way, liquor may be showered into the accumulator to absorb the gas and condense the vapors entering by way of pipe Il, .or the pump l5 may be intermittently operated so as to place a pool of acid liquor in the accumulator -into which the relief fluid from the high pressure header is introduced from the distributor Ila.

The acid liquor from the preparation apparatus may enter my system by way of a pipe line 23 which is provided with the usual settling tank 2|, and this liquor is forced by a pump 22, through a pipe .23 that terminates in a nozzle 24 which discharges the liquor into a clos/ed substantially cone-shaped chamber 25 arranged at the top of a down pipe 2B. This pipe also terminates in an ejector nomle 21 which enters adistributor tube 28, provided at its lower end with a perforated distributor 23; the parts 28 and 23 being arranged within the acid storage tank, so that a pool of liquor may be maintained inthe tank. While the ejector nozzle 21 is shown within the tank. it may be positioned outside the same.

A conduit 35, having a control valve 3|.'leads from the low pressure header 2 to an inlet 32 arranged on the side of the distributor tube, ad-

jacent the injector nozzle 21, whereby liquor 75 closed, and this gas is introduced into the liquor Y forced into the storage tank by means of the pump During the mung or the digester' with not mm a pipe 35 which leads to. the suction or discharge i5` n side of the gas fan, supplying S02 burner gas to the acid preparation system.

As some gas may be released in the chamber 25, I connect the'top of-this chamber by a pipe 35 to the pipe 35, so that this gas may alsobe fed tothe suction or discharge side of said gas fan.

In one method of operating, relief liquor instead of being discharged through the pipe 9,

travels through the branch 8' to a valved conduit l5' 31 which`leads the same into the upper portion of a relief tank 38 which preferably has a ca.-4 pacity about one-third as large 'as that of the digester.

As' will be hereinafter explained, hot strong 20 relief liquor from the tank 38 may be returned' to the digester by means ofa valved pipe 39, which is connected to the valved branch 8. A pipe 53 extends downwardly directly'below pipe l9, with valved branches 54, 55, 55 and 51 leading to the 25 digester at various elevations, whereby while the digester is beinglled with chips, the liquor from' the relief tank 38 may be admitted at different points along the vertical .height of the digester.

For instance, when the lling of the digester is first started, valve 51 will be opened and then I after the chip level has raised beyond valve 51, that valve will be closed, and valve 55 will be opened, and so on until the end of the filling operation, when the relief liquor will be going in through valve 52.' For the purpose of relieving the tank 38 of gases and entrained vapors, and to save such fluid and the heat umts impounded therein, I connect the top of the tank 38 to the lower portion of the accumulator by means of a pipe 40, and in this pipe, I interpose a pressure relief valve Il which is set to open at a predetermined pressure, whereby, when the pressure inthe tank 38 rises to this predetermined level, the gases and vapors will be automatically relieved from the liquor relief tank, and ow into the accumulator to Vbe absorbed or condensed by the liquor in the latter. As the pressure rises in the acumulator, it will be n to relieve the ,same of such pressure, so as to prevent this Ipressure from rising above a predetermined To accomplish such relief, I connect the top of the accumulator by means of a conduit 42. tothe pipe 28 which enters the suction side of the pump 22, and in theconduit 42, I arrange a pressure controlled valve 43.

From the above it will be seen that acid gas' from the digesters may now through the pipe 38 and be recovered by the liquor in the storage tank I 4, and in this way the liquor is strengthened and heated to some extent. Then this preconditioned and preheated liquoris .forced by the pump I8, into the accumulator to. be further strengthened and heated byrelief fluids from the high pressure header, and4 consequently, the e5 strengthened liquor is heated to a temperature say ranging from C. to 130' C., and after the A liquor has been'prepared in `this way, itlis fed fromthe bottom of the vaccumulator .by means of a pump 4l interposed in aconduit 45 which has control valves ll5 and l1 arranged respectively adjacent-to the lower lends of the digester and accumulator. In this way, the hot acid .'liquor is forced into'the digesterafter the latter has been charged with chips or other raw material. 7l

Each digester may have lthe conventional valved steam inlet means 43, and the valved outlet pipe 43, the latter leading to the blow-pit not shown.

In one method of operating in accordance wi the invention, and assuming that acid liquor in' the accumulator has been strengthened and preheated in the manner heretofore described, and that the digester is empty. At such time, the valve 46 andthe valves in the pipes 4, 5, 1, 3, 31, 33, 43 and 43 are closed. Wood chips or other brous material are then fed into the top of the digester until they build up to a level in close proximity to the top of the digester. Nowthe cover of the digester is fastened in place, the air valve vent i is opened, and the valves 46 and 41 are opened,and the pump 44 is started, so as to force the hot strong acid liquor from the accumulator into the digester. As soon as all air is discharged from -the digester, and SO: gas appears, the valve 6 is closed, and the valve in the branch 4 is opened. At'this time, the valve 3i is also closed, and the valve 34 is opened, so that the gas relieved from the digester ows into the pool of liquor in the tank I4 to be absorbed thereby. Any gas remaining unabsorbed, goes by way of pipe 35, back to the acid preparation system, as heretofore mentioned.

As soon as all of the, acid gas has been discharged from thedigester, and liquor begins to flow through the branch 4, the valve in that branch, and the valve 34 are closed, so that the entire upper end of the digester is now sealed, but the pump 44 is permitted to ontinue to operate, and it is kept running untilthe digester is not only completely lled by the liquor and chips, but until a desired pressure has been pumped on the digester. I have found in actual practice, the digesting process is immediately started when the digester is lled with acid of a temperature of 40 C., or over, and when I say this, I mean' -that the hot acid puts into partial solution the non-cellulose components of the raw material which are commonly called water solubles, and this allows a more thorough and rapid penetration of the brous material.

As soon as the pressure has been pumped on the digester, steam is admitted to same through the pipe 4B. The digester is brought upA to a pressure of from 50 to '15 pounds in from one to three hours, varying in respect tov the length of the cook. After the desired pressure has been reached, the top relief valve in branch 50, and valve I are partially opened and relief is carried on, until an approximate temperature of 115 C. has been reached, then the top relief is closed, and the side relief valve 52 in branch 8 is partially opened, then the digester is relieved until an approximate temperature of 125 C. has been reached. Then the side relief valve is closed', and the top relief valve in branch 50 opened. If the top relief valve picks up liquor, the side relief is again opened-for further drainage. The temperatures cited-will vary considerably in relation to the length of cooking time and strength of acid.

My reason for bringing the liquor through the top relief 1 until a temperature of 115 C., has been reached, is because the admittance of steam in the bottom of the digester condenses and makes additional liquor, and as this liquor is forced through the top of the digester, I make the same travel through, and therefore, come in contact with more of the raw material contents of the 75 digester. Consequently, I make the liquor do the steamis turned intothe same, I am able toA more evenly distribute the heat units and keep the fibrous material more thoroughly saturated by relieving through the top relief than I would if I relieved through the side relief Il.

I have found with a standard size digester, that v by the time I have relieved all the liquor from a. level above the side relief I, (the latter being 8 to feet below the top flange of the digester) the temperature in the digester rises to 125 C., owing to the continued relieving of gases, and the introduction of steam. As soon as the temperature 125 C. has been reached, and the liquor level has been brought down so that it is approximately eight feet below the top of the digester, the side relief branch 3 is closed, and the top vhigh pressure relief branch 1 is again opened, so that gases and vapors are relieved through the header 3 and pipes I0 and II, until the pressure `between the digester and the accumulator is equalized, whereupon the valve in the branch 1 is closed, and the valve in -the branch 4, and the `valve 3l are again opened, whereby the low pressure relief fluid passes into the tube 28 where it is mixed and absorbed by the liquor discharging from the injector nozzle 21.

In this way, liquor in the storage tank I4, and the accumulator I2, is strengthened, and the liquor in the accumulator is highly heated, so that liquor thus preconditioned may be fed into another digester which has just been charged with chips, and the operation may be again repeated.

In an alternative method of operation the system, the digester is lled with chips in the usual manner, and then the valves 46, 41 are opened, and the pump 44 is started, whereupon hot fully conditioned acid is taken out of the accumulator and pumped into the digester. At such time, all of the digester connections, with the exception of the one 5, are closed, and consequently, as the liquor flows into the digester, the air escapes. As soon as the air has been discharged, the valve 6 is closed, and any SO2 gas now escaping, due to the filling of the digester, is permitted to flow through the pipes 3|! and 33, into the liquor in the tank I4. When the digester is completely filled with liquor and chips,'the valve 34 andthe valve in the branch 4 are closed, and the pumping is continued until the required pressure has been placed on the digester. Then the acid illiing pump 44 is stopped and the valves 46 and 41 are closed, and if the accumulator is filled inter.- mittently, the pump I6 is started, and the valves I1 and I9 are opened, and the accumulator is filled to the required height with partially conditioned acid from the tank I4.

As soon as the pressure has been pumped on the digester, the steam is admitted through the pipe 43, and after a suitable time, the valve in the pipe 31 is opened, so that liquor, due to the pressure existing in the digester, flows into the vliquor relief tank 38. either through pipesV 50, Il,

and the valve in the branch 1 is left open so that the dry gas passes through the header 3 and pipes I and Il, into the accumulator where it acts to heat and enrich thev acid liquor contained therein, and thus prepares such liquor for use in the digester which is to be lled.

At the last end of the cook, the pressure on the digester is brought down to a point where it equalizes with the pressure in the accumulator. and the pressure in the latter is kept at a maximum of 35 pounds, by the pressure operated valve 43. Then, when the pressures in the digester and accumulator are equalized, the valve vin the branch 1 is closed, and the valve in the branch l. as well as valve 3|, are opened, whereby low pressure gases and vapors may be sucked out through the pipe 3 0 by means of the jet of acid liquor discharging from the injector nozzle V21.

iThis suction action is continued until the pres-4 sure in the digester has been lowered sufllciently to permit blowing of the digester, then the valve in the branch l a'nd the valve 3| are closed, and

- the valve in the pipe I9 is opened, and the contents of the digester are pumped into the blow-pit.

The tank 38 is heat insulated, and during the cooking operation. the valve Il is set'so as to maintain the proper pressure in saidtank. Whenever the pressure rises above a predetermined point, the valve 4I opens and permits the released gas to flow over into the accumulator, through the pipe 40.

As soon as the digester has been emptied, it is again immediately filled with chips or other l fibrous material, and during the time of filling.

the valve in branch 31 is closed, and the valve in branch 39 is opened.`V Consequently, the hot acidrelief liquor from tank 3B is showered back through pipe 53 on top of the chips as'they fall into the digester, thereby accomplishing two results. First, this hot liquor returning under pressure into the digester, sprays over the chips and packs them more solidly in the digester, thereby taking the place of numerous steam devices previously used for this pin'pose. Second, the hot liquor spraying into the digester through branches 51, 55, 55, 54, 52, conditions the chips so that they are very nearly ofthe same moisture content, and in addition. as hot liquor is used for this purpose, partial penetration is immediately accomplished and a' good many of the so-called water solvents are removed from the chips. All of this means that more chips are placed in the digester, considerably more production is made with every cook,- and in addition, the chips are conditioned so that more uniform results are obtained, as wel] as action is started in dissolving the non-cellulose material in the fibrous material processed;

Each of the digesters of the plant may be equipped with a relief tank 38, or where there' are many digesters in the plant, a single relief tank can be used for as many digesters as the cooking schedule will permit.

I believe I am the first to separate the relief liquor from the main pool of liquor in the digester, and to conduct the hot gases from such relief liquor, into a. pool of liquor under pressure, and tothen reuse the hot relief liquor for packing chips -in the digester, and also for starting the dissolving action and conditioning the chips for further treatment. It is also novel, I believe. to'

have the overhead line I2 from the accumulator connected to the suction side of the pump 22 which forces the liquor to the acid storage tank. It will also be clear to those skilled in the art that ,I depart from the usual procedure in a number iol importantparticulars, and in doing so, I gain many advantages over the systems heretofore While Ihave disclosed1what I now consider to i be a. preferred embodiment ofthe invention in such manner that the same may be readily understood by those` familiar with the digesting of fibrous material, I am aware that changes may be made in the details disclosed, without departing from the spirit of the invention, as expressed in the claims.

vWhat I claim and desire to secure by Letters Patent is:

1. In the digestion of brous material, introducing acid liquor into a digester while discharging air to the atmosphere from the digester through an air vent until acid gas appears, and

asl soon as said gas commences to escape from the completely filling the remaining space in the digester with acid liquor at a temperature of 40 C. or more by pumping said liquor into the digester.

4. In the digestion of fibrous material, charging a digester with such material, and then completely iillingihe remaining space in the digester with acid liquor at a temperature ranging from 40 C. to 135 C., both inclusive, by pumping said liquor into the digester.

5. In the digestion of fibrous material, com.

pletely filling a digester with brous material and hot chemical liquor, sealing the digester, and

then pumping hot chemical liquor into the di- `gester until the contents of the digester are placed under super-atmospheric pressure.

6. I n the digestion of fibrous material, completely filling a digester with fibrous material and hot chemical liquorsealln'g the digester, and then pumping hot chemical liquor into the digester until the contents of the digester are placed under super-atmospheric pressure of at least l5 pounds.

"I. In the digestion of fibrous material, charging a digester with such material, then introducing acid liquor into the digester to completely ll the latter, discharging air from the digester during the filling until acid gas commences to escape, then recovering the acid gas until the digester is completely filled with liquor, then sealing the digester and pumping acid liquor into the digester until the contents thereof reaches a superf-atmospheric pressure.

In the digestion of brousmaterial, charging a digester with such material, then introducing acid liquor into the digester to completely fill ythe latter, discharging air from the digester during the filling until acid gas commences to escape. then recovering the acid gas until the digester is completely filled with liquor, then sealing the digester and pumping acid liquor into the digester until the contents thereof reaches a super-atmospheric pressure of up to 300 pounds.

9. In the digestion of fibrous material, charging a digester with such material, then introducing hot'acid liquor into the digester to completely fill Athe latter, discharging air from the digester during the filling until acid gas commences to escape, then recovering the acid gas until the digester is completely lled with liquor, then sealing the digester and pumping acid liquor into the digester until the contents thereof reaches a super-atmospheric pressure.

10. In the digestion of fibrous material, charging a digester with such material, then introducing hot acid liquor into the digester to completely fill the latter, discharging air from the digester during the filling until acid gas commences to escape, then recovering the acid gas until the digester is completely filled with liquor, then sealing the digester and pumping acid liquor into the digester until the contents thereof reaches a super-atmospheric pressure of up to 300 pounds.

11. In the digestion of fibrous material, completely filling a digester with fibrous material and chemical 1liquor, sealing the digester and then pumping acid liquor into the digester until the contents of the digester reaches a pressure above 15 pounds, then discontinuing the pumping and introducing steam into the digester, and at about4 the same time releasing acid liquor from the digester and contacting the released acid liquor with other acid liquor to recover the released liquor.

12. In the digestion of fibrous material, completely filling a digester with fibrous material and hot acid liquor, sealing the digester, then pumping acid liquorI into the digester until a pressure above 15 pounds is reached in the digester, then discontinuing the pumping and introducing steam into they digester, and about the time of the introduction of the steam, releasing acid liquor from the digesterand introducing the released liquor into a pool of acid liquor contained in a pressure accumulator.

13. In the digestion of fibrous material, completely filling a digester with fibrous material and acid liquor having a temperature within a range of 40 C. to 135 C., sealing the digester, pumping hot acid liquor into the digester until the pressure'therein` reaches up to 300 pounds, then discontinuing the pumping and introducing steam into the digester, and subsequently passing hot relief fiuid from the upper portion of the digester into acid liquor in a pressure accumulator.

14. In the digestion of fibrous material, com# pletely filling a digester with such material, and hot acid liquor, discharging during the filling, acid gas from the upper portion of the digester, into fresh acid liquor'in -a storage tank, sealing the digester afteritis completely filled, then pumping hot acid liquor into the digester until a pressure of up to 300 pounds is reached in the digester, then discontinuing the pumping and introducing steam into the digester, and subsequently passing relief fluid from the upper portion of the digester into acid kliquor in a closed accumulator;

l5. In a process of the character described, discharging hot relief uid from a digester into acid liquor in a pressure accumulator, pumping acid liquor to a'storage tank, and utilizing the pump- 75 pumped to the storage tank.

16. In a process of the character described, passing hot relief fluid from a digester undergoing and mixing the same with acid liquor in a pressure accumulator, forcing a stream of fresh acid liquor to a storage tank, and releasing acid gas from the accumulator when a predetermined pressure is reached in the latter, and introducing the last mentionedl gas into the stream of fresh acid liquor flowing to the storage tank.

17. In a process of the character described, utilizing a stream of acid liquor forced into a storage taik to withdraw relief fiuids from a digester and a pressure accumuiatorfand to mix the withdrawn fluids with the pumped acid liquor.

18. In a process of the character described, passing hot relief fluid from a digester undergoing cooking, into acid liquor in a pressure accumulator, pumping fresh acid liquor to a'storage tank, utilizing the pump to mix acid gasreleased from the accumulator with the liquor flowing to the storage tank, and pumping acid liquor from the storage tank into the accumulator.

19. In a process of the character described, passing hot relief fluid from a digester undergoing cooking, into acid liquor in a pressure accumulator, pumping fresh acid liquor to a storage tank, utilizing the pump to mix acid gas released from the accumulator with the liquor owing to the storage tank, pumping acid liquor from the storage tank into the accumulator, and utilizing hot acid liquor from the accumulator for digesting fibrous material.

20. In a process of the character described, completely filling a digester with hot acid liquor and fibrous material, then releasing hot acid liquor from the digester at the top of the same, and introducing the released liquor into a pressure tank, and subsequently using the relief liquor from said pressure tank to pack fibrous material into the digester during the filling of the latter and to precook the fibrous material as it is introy terial, allowing this relief liquor to fiow into anelevated pressure tank, and subsequently allowing said hot relief liquor to ow from said tank under the pressure existing in the latter, into the digester while charging the latter with fibrous material, whereby said brous material is precooked and packed into the digester.

23. In a process of the character described, permitting hot acid liquor to fiow from the upper portion of a digester, into an elevated relief pressure tank, and subsequently utilizing bot relief liquor from said tank by flowing said liquor into the digester while filling the latter with fibrous material, whereby said fibrous material is precooked and packed into the last mentioned digester.

24. In a process of the character described, completely lling a digester with fibrous material and hot chemical liquor, then owing hot chemicall liquor through the relief branch of the digester into an elevated pressure tank, automatically releasing acid gas from said tank and contacting the same with acid liquor to recover said gas, and introducing hot relief liquor from said tank into the upper portion of a digester' while charging the latter with brous material, whereby said fibrous material is precooked and packed into the digester.

25. In an apparatus of the character described, an acid liquor storage tank, a pipe line having an interposed pump by means of which.fresh acid liquor is forced into said storage tank a digester and a valved conduit connecting the upper portion of the digester to said pipe line, whereby liquor flowing through the pipe line acts to withdraw relief uid from the digester through said conduit. 26. In an apparatus of the character described, a digester and-'an acid liquor storage tank, an acid liquor conveying pipe 4extending into the -storage tank, a conduit connecting the upper portion :of the digesterto said liquor conveying pipe, a control valve for the conduit arranged adjacent to the upper portion of the digester, a valved branch connecting'said conduit to the storage tank, and a valve interposed in the conduit between the branch and said liquor conveying pipe.

27. In an apparatus of the character described. a digester provided-with a low pressure relief branch. a high pressure relief branch, and a side relief. branch, and valved means for venting air from the low pressure relief branch into the atmosphere.

28. I n an apparatus of the character described, a digester, an elevated side relief tank, avalved conduit connected to the side of the digester. and valved branches connecting said conduit to upper and lower portions oi said tank. I

' 29. In an apparatus of the character described, a digester having a valved side relief conduit, an elevated tank having top and bottom valved branches connected to said conduit, the major portion of said tank being arranged above said conduit, and a valved pipe connecting the top of the digester to said conduit.

30. In an apparatus of the character described.

-a digester having a valved side relief branch, an

elevated pressure tank connected to said branch, and a valved conduit connecting said branch to the upper portion of said tank.

3l. In an apparatus of the character described, a digester having a valved side relief branch, an elevated tank connected to said branch, a relief pipe for said tank, and a pressure. operated valve arranged in the relief pipe.

32. In an apparatus of the character described, a digester having a valved side relief branch, an elevated tank connected to said branch, a. pressure accumulator, a relief pipe connecting the upper portion of the tank to the lower portion of the accumulator, and a pressure operated valve lnterposed'in the relief pipe.

33. In an apparatus of the character described, an acid liquor storage tank, a pipe line for conveying acid liquor to said storage tanka pump interposed in said pipe line, a pressure accumulator, a conduit connecting Ythe upper portion of the accumulator to the pipe line adjacent the suction side of the pump, and a, pressure operated valve interposed in said conduit.

34. In a process of the character described, lilling afdigester with hot acid liquor and fibrousv material. releasing hot acid gas and/or liquor from the upper portion of the digester and introducing the released fluid into an elevated pressure tank, and subsequently using the relief liquor from said pressure tank to pack fibrous material in a digester during the lling of the latter with such brous material.

35. In the digestion of fibrous material, illling a digester with hot acid liquor and nbrous material under pressure. releasing acid liquor from the top of the digester and introducing the same into an elevated pressure tank, releasing acid liquor through the side relief of the digester and depositing the same in said pressure tank, and subsequently using'hot relief liquor from said tank to pack fibrous material into a digester and to precook the last mentioned ilbrous material.

36. In the digestion of fibrous material, lling a, digester with hot acid liquor and brous material, releasing acid iluid from the upper portion of the digester and introducing the same into an elevated pressure tank, and subsequently feeding hot relief liquor from said tank-into a digester at vvarious elevations while depositing fibrous material into'the last mentioned digester for the purpose of packing and precooking the fibrous material in the last mentioned digester.

37. An apparatus for'digesting fibrous material, including a digester having a valved top relief branch and a valved side relief branch, an elevated liquor relief tank, a conduit connecting said branches to said tank,and a pressure operated valve for releasing gas from said tank.

38. Anapparatus for digesting fibrous material, including a digester, an elevated liquor relief tank. valved means connecting the upper portionof the digester to said relief tank, a conduit depending from said means, and valved branches arranged at different elevations and connecting said digester to said conduit.

39. In the digestion of brous material the method which comprises completely filling the digester containing the brous charge with liquor under a pressure at'least approximating the desired cooking pressure by pumping said liquor into the digester, and introducing steam thereto to eect the cook. while substantially maintaining such pressure.

V 40. In the digestion of fibrous material the method which comprises completely filling the digester containing the ilbrous charge with acid liquor under a pressure at least approximating the desired cooking pressure, discharging a portion of the liquor without substantial fall of such pressure and introducing steam thereto to effect the cook while substantially maintaining such pressure.

41. In the digestion of ilbrous material 'the method which comprises saturating the chips with acid liquor by an elevated pressure applied through a column thereof and thereupon introducing steam to eiect the cook while maintaining a pressure substantially constantk throughout the cooking period.

42. In the digestion of nbrous material the method which comprises saturating the chips with acid liquor by an elevated pressure pumped thereon through a column of said liquor, introducing steam to eect the cook while maintaining a pressure of several atmospheres and regulating the cooking pressure by relieving the same to maintain a substantially constant value during f the cook.

1 43. In `the digestion of fibrous material the method which comprises saturating the chips with liquor at a pressure substantially exceeding the cooking pressure and exerted by pumping applied through a column of the liquor and introu ducing steam to eect the cook while maintaining the pressure at least substantially at the desired cooking pressure and maintaining such pressure substantially .constant throughout the period of the cook.

44. In the digestion of lbrous material, completely filling a digester with brous material and acid liquor while discharging air from the digester, then sealing the digester, and afterwards pumping acid liquor into the digester until the contents of the digester are placed under superatmospheric pressure.

45. In the digestion of fibrous material, completely filling a digester with brous material and acid liquor while discharging air from the digester, then sealing the digester, and afterwards pumping acid liquor into the digester until the contents of the digester are placed under a super-atmospheric pressure of at least l5 pounds.

46. In the digestion of fibrous materiaLrcharging a digester with such material, then forcing acid liquor into the digester to completely fill the latter, discharging aeriform fluid from the digester during the filling, then sealing the digester and pumping acid liquor into the digester until the contents thereof reaches a super-.atmospheric pressure.

47. In the digestion of fibrous material, that improvement which comprises charging a digester with brous material, then introducing into the digester a displacement medium including other gases and thereby driving of! the air and inert gases from the digester into the atmosphere, and then subjecting the brous material in the substantially de-aerated digester to digestion with a cooking liquor. A

48. In Athe digestion of fibrous material, that improvement which comprises charging a digester with fibrous material, then displacing the air and inert gases from the digester by introducing into the digester a displacement fluid in'- 'cluding SO2 gases, utilizing said SO2 gases to de-aerate the brous material in the digester and to drive off air and inert gases to the atmosphere,

and finally subjecting the brous material in the substantially de-aerated digester to digestion with acid liquor.

49. In the digestion of fibrous material, charging a digester with ilbrous material, then introducing hot bisulphite acid liquor into the lower v end of the digester and permitting acid gas to escape from the liquor in the digester, utilizing said gas to displace air and other inert gases from the digester and its contents during the introduc tion of the liquor and thereby permitting the -the latter a liquid displacement medium including another gas, utilizing the last-mentioned gas to de-aerate the fibrous material and to drive oi air and inert gases to a region of lowerpressure then is used in the `digester during the cooking operation, and finally subjecting the fibrous material to digestion with a cooking liquor. 4

THOMAS L. DUNBAR. 

